[llvm-bugs] [Bug 40483] New: [X86] Failure to merge ISD::SUB(x, y) and X86ISD::SUB(x.y)

via llvm-bugs llvm-bugs at lists.llvm.org
Sat Jan 26 13:38:31 PST 2019


            Bug ID: 40483
           Summary: [X86] Failure to merge ISD::SUB(x,y) and
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: llvm-dev at redking.me.uk
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

We sometimes fail to recognise that ISD::SUB(x,y) and X86ISD::SUB(x,y) can be
represented by the X86ISD::SUB(x,y).


#include <x86intrin.h>

using u64 = unsigned long long;

u64 test_sub1(u64 *p, u64 x) {
  u64 r = *p - x;
  _subborrow_u64(0, *p, x, p);
  return *p - r; // zero

u64 test_sub2(u64 *p, u64 x) {
  u64 r = *p - x;
  _subborrow_u64(0, *p, x, p);
  return r - *p; // zero

        movq    (%rdi), %rcx
        movq    %rcx, %rax
        subq    %rsi, %rax
        movq    %rax, (%rdi)
        subq    %rcx, %rsi
        addq    %rsi, %rax
        subq    %rsi, (%rdi)
        xorl    %eax, %eax

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